Usually vessels designed for holding gases and/or liquids, especially when the gases and/or liquids are stored under high pressure, are constructed out of metal. Gas cylinders or fire-extinguishers are examples hereof. These are vessels that consist of a steel (outer) casing, usually shaped in the form of a cylinder, having a steel neckpiece or fitting at one end. In the case of the pressurized gas cylinder, the material in such cylinder is separated from the outside environment by a stopper or plug that is screwed onto the fitting.
In the case of the fire-extinguisher, the pressurized cylinder is connected to a nozzle that, in case of an emergency, using a nozzle-mounted lever, sprays the contents of the extinguisher onto the source of danger which is in most cases a fire.
A first disadvantage of such metal/steel vessels is their weight. This is especially noticeable in vessels with a higher capacity, for example a fire-extinguisher with a capacity of more than 10 kg. Their large weight makes such high capacity vessels difficult to handle and limits their usability in real world applications.
Because a large proportion of these vessels have to be used in cases of an emergency, where weight and consequently ease of use is crucial, this disadvantage becomes more pronounced. A second disadvantage of these metal/steel vessels is their price, dictated by the high manufacturing costs. A third disadvantage is rust. This occurs for example in the case of fire-extinguishers as a result of the interaction between the metal casing and the fire-extinguishing contents. The rust consequently is formed on the inside of the vessel and is thus not noticeable from the outside. Rust forming in fire-extinguishers is especially noticeable when they are filled with a particularity aggressive type of extinguishing agents such as a foam based extinguisher. When the amount of rust becomes substantive, the inside of the vessel can be corroded in such a way that, when it is used, the generated pressure can cause the vessel to explode in the operators hands.
WO 01/14212 A2 published Mar. 1, 2001, describes this problem and proposes a solution for this problem. The proposed solution consists of adding a second vessel within the metal vessel. This inner container consists of a thermoplastic synthetic material. As an example, a description is given of the procedure of adding such an inner container using blow-molding, in this case a pre-form or pre-product made of polyethylene terephthalate. This technique creates a vessel consisting of two layers with the inner layer acting as an effective protection against corrosion from a potentially aggressive fire-extinguisher material. The metal outer layer gives the vessel its actual strength. Using the blow-molding technique, the metal outer layer functions as the outer mechanical mold. In order to let the air between the synthetic pre-form and the metal outer layer escape during blow-molding, air escape holes have to be provided through the metal outer layer, connecting the inside of the vessel with the outside atmosphere.
This obviously reduces the strength of the outer layer and renders this technique for a practical application virtually unsuitable. For example, one of the principal requirements of a fire-extinguisher is to be able to contain the pressurized propellant for an extended period of time. In order to achieve this, mechanical strength is essential and by providing air escape channels this strength is greatly reduced.
Nevertheless, this technique is considered advantageous compared to other multi-layer-vessel techniques described by inventions disclosed as prior art. These techniques include adding an inner layer consisting of either a thermoplastic synthetic powder, a phenol-formaldehyde varnish or a multicomponent synthetic resin.
A further disadvantage of the traditional metal fire-extinguishers is that they can be potentially lethal, in the sense of becoming a potential bomb, under certain circumstances. These conditions occur e.g. when they are left unused in or near a fire. The heat from said fire can cause the fire-extinguisher to explode under extreme circumstances, placing rescue personnel such as fire-fighters in additional danger. In order to reduce the possibilities of these extreme circumstances causing an explosion, a vessel provided with thicker walls can be used. Such measure however increases the weight of the fire-extinguisher and this reduces its usability; so in practice the risk remains.
In view of the disadvantages mentioned above, it is recommended not to construct said vessels using metal/steel. In this case, a suitable synthetic material is more appropriate. Such vessels are not only lighter than their metal counterparts, but they are also cheaper to fabricate. Furthermore, synthetic containers will melt when exposed to high temperatures and thus reduce the potential risk to rescue personnel.
Methods for creating synthetic vessels using gas and/or leak-tight materials are known. WO 2008/119147 A1, published Oct. 9, 2008 in the name of Delgado Junior, describes pressure vessels for use in fire-extinguishers made from a single-layer composite material. This composite material consists of polyamide, glass fiber and minerals such as calcium, aluminum, . . . in addition to other elements such as UV protective additives. No actual method for manufacturing such vessels is described in this document.
Usually, gas and/or leak-tight pressurized vessels are constructed using multiple layers of material. The inner layer, usually made out of a thermoplastic material serves as a barrier layer, or as an impregnable layer for the gas and/or liquid. Around the inner layer, an additional (outer) layer is thermoset material, the arguments for which are provided below.
A known method consists of using an external mold or mandrel, containing a fitting or neckpiece, and using a technique known as roto-molding to apply a wall of thermoplastic material. After this step, the mandrel is removed and an additional wall, consisting of a thermoset material, is added around the thermoplastic inner wall.
Such a roto-molding technique is described in, for example in the French patent no. 1 520 457, granted Mar. 4 1968. The aforementioned patent describes a method for coating the inside of a vessel using a continuous layer of synthetic material. This inner layer is used to halt corrosion or to strengthen the vessel. Hereto roto-molding is used that can consist of placing the vessel on a device that rotates the vessel on a central axis. The synthetic material can then, for example in the form of a powder, be inserted into the vessel. The rotation of the vessel, combined with a heat source will create a continuous layer on the inside of the vessel.
After the manufacture of this inner layer, an additional or external layer should be added. During this second production step, the thermoplastic (inner)layer will serve as internal mandrel or mold.
The material of the external layer can be added through the use of various methods known per se: for example by winding of filaments or fibers, or by addition of according to size pre-cut materials, by spraying or by any other method. As material, always a thermoset synthetic material is chosen. Once the said material is added according to one of the methods described above, a polymerization takes place.
The addition through the use of a winding technique is one of the possible methods. In such a case, the inner thermoplastic layer should be sufficiently strong to avoid deformation during the winding of the outer fibers. To this end, it usually suffices to keep a minimum thickness, depending on the size of the envisaged vessel. The larger the vessel, the stronger the forces that occur during the winding of the thermoset fibers on the inner layer, and consequently the thicker the inner layer should be.
To avoid deformation of the inner thermoplastic layer during the addition of the thermoset-fibers to form the outer layer, one can also make use of a suitable counter-pressure in the inner layer.
Once the material of the outer layer is added through the use of such fiber-winding technique over the inner layer, the fibers should be attached or connected to one another. This occurs during a consolidation step. So as to avoid deformation of the inner layer (and as a result hereof the entire vessel) during this step, the material of the outer layer should necessarily be a thermoset. When such material is choses, the consolidation can occur through the application of cold polymerization.
Thus, as the inner thermoplastic wall that is formed in a first step should act as mandrel as well during the addition of the second outer wall material, as well as during the subsequent polymerization hereof, the material of this second wall or layer should necessarily be a thermoset material.
Such materials are indeed suitable for cold polymerization. The inner thermoplastic wall will then not deform, nor during the addition of the thermoset material of the second wall, nor during the subsequent polymerization step.
However, a double-layered synthetic material consisting on the one hand of an inner layer consisting of a thermoplastic material, and on the other hand of an outer layer consisting of a thermoset material, exhibits some disadvantages.
A first disadvantage is that a thermoplastic and a thermoset material inherently are incompatible, and thus do not yield one strong rigid structure.
In most cases the two layers remain one near to the other, without strong interaction between both layers.
A second disadvantage is that such materials cannot be recycled, or at least are difficult to recycle, in view of the fact that they consist of inherently incompatible materials.
As a result, there exists a long-felt need for a method for manufacturing multi-layered fluid-tight and/or gastight vessels for the holding of liquids and/or gases, that entirely consist of thermoplastic material.
In practice this was up to now either not possible, or quite complicated.
In a number of cases the material of the outer layer or wall is added through the winding of fibers or filaments that in a subsequent manufacturing step are molten one to another by heat.
This melting action by heat however gives rise during this production step to the fact that the inner thermoplastic layer that serves as a mall, by influence of this heat also weakens and consequently deforms. The so produced vessel then is unsuitable for use, for two reasons. On the one hand, this inner thermoplastic layer, as a result of such weakening and deformation, has lost its barrier-acting properties. As a result hereof, the vessel also has lost its gas and/or liquid impermeable properties, and hence becomes unsuitable for use. On the other hand, the final vessel is deformed, as as a result hereof also becomes unsuitable for use.
In the prior art solutions are provided to this problem, but these solutions are unsatisfactory in practice, in particular in the case of the manufacture of relatively small vessels such as pressure vessels for fire-extinguishers.
One such solution provides in the reinforcement of the inner thermoplastic wall from its inner side by using a soluble, inflatable, plaster or mechanical mold that can be disassembled.
French patent Nr. 2 173 837 discloses on its page 1, line 8-9 the use of inflatable internal mandrels. A synthetic layer can then be added to the surface of such mandrel for the manufacture of a vessel.
U.S. Pat. No. 3,220,910, granted Nov. 30, 1965, describes an example of a plaster internal mall or mandrel that can be used for the above purpose.
In practice this solution in particular in the case of relatively small vessels cannot be used. When such plaster mandrel is mechanically removed, the inner wall in most cases is damaged, as a result whereof the vessel appears to have lost its liquid and/or gas tight property. On top hereof, such a plaster mandrel can only be used once, as a result whereof the manufacturing cost of the so produced vessel becomes too high.
The use of a soluble mandrel also does not constitute a satisfactory solution. Either such method is unsatisfactory because parts of the partly solved mandrel remain in the so produced vessel, either such method appears to be expensive and time-consuming. U.S. Pat. No. 3,508,677 describes an example of such soluble mandrel, in particular reference is made to the FIG. 2 of this patent, wherein such a soluble mandrel is shown.
The method whereby an internal mechanically removable mandrel, e.g. consisting of metal parts, is used, is also a time-consuming and hence expensive technique. The difficulties relating to the use of such removable mandrel result from the fact that such mandrel is present inside the vessel. On top hereof such method can only be used in the case of larger vessels, or at least vessels whereby the size of the neckpiece or the fitting is sufficiently large so that the metal parts present inside the vessel can be removed from the vessel through the opening of such fitting.
U.S. Pat. No. 5,266,137 describes an example of such mechanical internal mandrel, that is kept in its correct position by using inflatable ‘balloons’. But even in this case the taking apart and removing through the opening of the fitting of the metal parts is a cumbersome activity. Also this should be performed with the necessary care to avoid damage to the internal thermoplastic wall. Even a small damage could lead in most cases to a situation whereby the finally produced vessel has lost its gas and/or liquid tight properties.
U.S. Pat. No. 4,448,628 describes also the use of such mechanically removable mandrels, and states that these can be used for the manufacture of large hollow vessels. Such vessels can be used for example as storage containers or as vessels for the storage of fuels for space vehicles, but are totally unsuitable for the production of smaller vessels.
The aim of the present invention is to bring about a satisfactorily solution to the abovementioned problems.